Crossing over in the tetrad. Two homologous chromosomes are duplicated in S phase to produce two sister chromatid pairs (left), one homozygous VV and the other homozygous vv. The sister chromatid pairs are homologous to each other, and they pair to form a tetrad in prophase I of meiosis, before the first cell division (middle). Recombination—as evidenced by the exchange of V and v alleles—occurs at crossovers, or chiasmata, the sites where chromosomes intertwine, resulting in genetic exchange between the two chromosomes of the sister chromatid pairs (right).